Conventional ultrasound scanners are capable of operating in different imaging modes. In the B mode, two-dimensional images can be generated in which the brightness of each display pixel is derived from the value or amplitude of a respective acoustic data sample representing the echo signal returned from a respective focal position within a scan region.
In the B-mode imaging, an ultrasound transducer array is activated to transmit beams focused at respective focal positions in a scan plane. After each transmit firing, the echo signals detected by the transducer array elements are fed to respective receive channels of a receiver beamformer, which converts the analog signals to digital signals, imparts the proper receive focus time delays and sums the time-delayed digital signals. For each transmit firing, the resulting vector of raw acoustic data samples represents the total ultrasonic energy reflected from a succession of ranges along a receive beam direction. Alternatively, in multi-line acquisition two or more receive beams can be acquired following each transmit firing.
In conventional B-mode imaging, each vector of raw acoustic data samples is envelope detected and the resulting acoustic data is compressed (e.g., using a logarithmic compression curve). The compressed acoustic data is output to a scan converter, which transforms the acoustic data format into a video data format suitable for display on a monitor having a conventional array of rows and columns of pixels. This video data is referred herein as “raw pixel intensity data”. The frames of raw pixel intensity data are mapped to a gray scale for video display. Each gray-scale image frame, hereinafter referred to as “gray-scale pixel intensity data”, is then sent to the video monitor for display.
A conventional ultrasound imaging system typically employs a variety of gray maps, which are simple transfer functions of raw pixel intensity data to display gray-scale values. Multiple gray maps are supported so that different maps may be used depending on the range of pixel intensities. For example, if a given application tends to generate mainly low raw pixel intensities, then a gray map which dedicates more gray-scale values to low raw pixel intensity values is desired since it improves the contrast across this range. Therefore, it is typical to default to a different gray map depending on the application. However, this is not always effective since the user can scan any anatomy in any application, acoustic data varies from patient to patient, and the raw pixel intensity values depend on other system settings such as dynamic range and gain. Due to these factors, the gray maps tend to be conservative with respect to how many gray-scale values are dedicated to the anticipated primary pixel intensity range.
The default values of many ultrasound image processing parameters, such as dynamic range for two-dimensional image display, transmit foci positions, gain, etc., are preset in a preset menu. These preset values are often based on clinical testing aimed at achieving optimal image performance for the average or typical patient in a particular exam or application type. However, for certain classes of image processing parameters such as those affecting the display contrast, different users may have different preferences depending on application type, and furthermore, their preferences may evolve over time as they develop experience with the scanner.
In conventional scanners, there are also many image processing parameters that are not available in the preset menu; i.e., they may not be adjustable by the user. One example is the adaptive gray mapping feature for B-mode imaging, as taught in U.S. Pat. No. 6,048,311 entitled “Method and Apparatus for Ultrasound Imaging Using Adaptive Gray Mapping.” This reference teaches a method which allows the system user to adjust the contrast by pressing a button on an operator interface. When the user has positioned the probe over the anatomy of interest, depressing a button triggers the host computer inside the ultrasound imaging system to retrieve the current frame of raw pixel intensity data, analyze its pixel intensity histogram within a user-specified region of interest (ROI), and then automatically scale and/or shift the gray mapping (i.e., raw pixel intensity to gray-scale pixel intensity mapping) such that pre-defined (i.e., fixed by the manufacturer) optimal upper and lower gray-scale levels map to some upper and lower bounds of the pixel intensity histogram respectively. The ultimate goal is to more fully utilize the available gray-scale levels (256 levels for an 8-bit display system) to display the pixel intensity data, thereby improving the display contrast.
The limitation with the foregoing approach is that no predefined goodness criteria, in terms of gray level limits, will satisfy all users. If the user finds the adaptive gray mapping gives too much contrast, he/she would often manually change the dynamic range or gain to counteract the gray-scale changes effected by the adaptive gray mapping feature. One solution is to make the target gray map parameters adjustable in the preset menu. But as stated above, user preferences may change with time and having additional adjustable parameters may detract from the adaptive power of the adaptive gray mapping feature.